A typical wireless communication system includes one or more base stations, each radiating to define one or more cells in which wireless client devices (WCDs) such as cell phones, gaming devices, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped devices can operate. Further, each base station of the system may then be coupled or communicatively linked with network infrastructure such as a switch and/or a gateway that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a WCD within coverage of the system may thus engage in air interface communication with a base station and thereby communicate via the base station with various remote network entities or with other WCDs served by the system.
Such a system may operate in accordance with a particular air interface protocol or radio access technology (RAT), with communications from a base station to WCDs defining a downlink or forward link and communications from the WCDs to the base station defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Long Term Evolution (LTE) (using Orthogonal Frequency Division Multiple Access (OFDMA) on the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) on the uplink), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communication (GSM), WI-FI, and BLUETOOTH. Each protocol may define its own procedures for registration of WCDs, initiation of communications, handover between cells, and other functions related to air interface communication.
In accordance with the air interface protocol, each of the one or more cells of the system may operate on a respective carrier, which could be frequency division duplex (FDD) or time division duplex (TDD), defining one or more frequency channels each typically within an industry standard frequency band. An FDD carrier defines separate frequency channels for downlink and uplink communication, so that WCDs can engage in downlink and uplink communication. Whereas, a TDD carrier defines a frequency channel multiplexed over time (e.g., on a recurring basis) between downlink and uplink use, so that WCDs can engage in downlink communication in only particular time intervals and uplink communication in only other time intervals.
Each carrier in such a system may also define a number of sub-channels for carrying information between the base station and WCDs. For instance, each carrier may define a pilot channel, reference channel or other resource on which the base station can broadcast a reference signal or the like that WCDs may detect as an indication of coverage and may measure to evaluate coverage. Further, each carrier may define various other downlink and uplink control channels and traffic channels for carrying control signal and bearer traffic between the base station and WCDs.
When a WCD initially enters into coverage of a wireless communication system (e.g., powers on in coverage of the system), the WCD may scan for a strongest (e.g., highest signal-to-noise ratio) reference signal, defining a strongest cell. Further, the WCD may read system information broadcast from the base station that provides the cell and may engage in a process to register itself to be served by the base station and generally by the system. In this process, for instance, the WCD and base station may first engage in signaling to establish a radio-link-layer connection between the WCD and the base station, the WCD may transmit an attach request or the like, and the base station and/or supporting infrastructure may authenticate and authorize the WCD for service, and establish a record indicating where in the system the WCD is operating. The WCD may then be served by the base station in an idle mode or a connected mode.